System and method for increasing radio frequency (RF)/microwave inductor-capacitor (LC) oscillator frequency tuning range

ABSTRACT

System and method for increasing the frequency tuning range of a RF/microwave LC oscillator. A preferred embodiment comprises a voltage controlled oscillator (VCO) configured to generate an output signal at a frequency that is dependent upon a magnitude of an input voltage level and an effective inductance of an inductive load and a variable inductor coupled to the VCO. The variable inductor comprises a primary inductor coupled to the VCO to produce a magnetic field based upon a current flowing through the primary inductor and a secondary inductor magnetically coupled to the primary inductor, the secondary inductor to affect the magnitude of the effective inductance of the primary inductor.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of U.S. Provisional Application No.60/546,248, filed Feb. 19, 2004, entitled “A Novel and Economic Way toIncrease an RF/Microwave LC Oscillator Frequency Tuning Range,” whichapplication is hereby incorporated herein by reference.

TECHNICAL FIELD

The present invention relates generally to a system and method forwireless communications, and more particularly to a system and methodfor increasing the frequency tuning range of a RF/microwave LCoscillator.

BACKGROUND

A desire of a cellular telephone user is to have a single cellulartelephone that can operate anywhere in the world, a “world” phone.Unfortunately, in order to operate at different locations throughout theworld, the world phone needs to be able to communicate with differentcellular communications networks, each using a potentially differentcommunications mechanism, as well as being able to tune its oscillatorto different frequency ranges. This is due to the wide variety ofcommunications standards being used in different nations. For example,in the United States, it is common to encounter GSM (Global System forMobile Communications), CDMA (Code Division Multiple Access), and TDMA(Time-Division Multiple Access) cellular telephone networks. In additionto the three types of cellular telephone networks, there can be multiplecommonly used frequency ranges, 850 and 900 MHz ranges as well as 1.8and 1.9 GHz ranges. In other countries, other types of cellulartelephone networks and frequency ranges may be in use.

One solution to the problem of being able to tune an oscillator to sucha wide variety of frequency ranges is to have multiple oscillators, onefor each frequency range of interest. With multiple oscillators, eachoscillator can be optimized for each frequency range, potentiallymaximizing tuning accuracy.

Another solution to the problem is to use a single oscillator but withmultiple oscillator cores or LC tanks. The multiple oscillator cores orLC tanks can be used to extend the tuning range of the single oscillatorwithout needing multiple oscillators. Each of the multiple oscillatorcores or LC tanks can be switched in when needed. Again, the use ofmultiple oscillator cores or LC tanks can allow the optimization for thedifferent frequency ranges.

Yet another solution to the problem is to use a switch, such as a CMOSswitch, to short circuit a portion of an inductor to increase the tuningrange of the local oscillator. The use of the switch can permit the useof a single local oscillator.

One disadvantage of the prior art is that the use of multiple localoscillators and/or multiple oscillator cores or LC tanks is that theoscillators consume a considerable amount of silicon area. Therefore, itis desired to minimize the number of local oscillators or oscillatorcores.

A second disadvantage of the prior art is that due to a high qualityfactor requirement for the inductor used in the oscillator, the power onresistance of the switch must be low. This places a requirement that theswitch must be physically large, thereby requiring a large amount ofsilicon area to be dedicated to the switch and the oscillator.Furthermore, as the oscillator operating frequency increases, parasiticcapacitance introduced by the large switch can become problematic due tothe fact that the total capacitance of the oscillator core is small.

SUMMARY OF THE INVENTION

These and other problems are generally solved or circumvented, andtechnical advantages are generally achieved, by preferred embodiments ofthe present invention which provides a system and method for increasingthe tuning frequency range of a RF/microwave LC oscillator.

In accordance with a preferred embodiment of the present invention, anintegrated circuit comprising a first inductor formed on a first layerand a second inductor formed on a second layer and sharing asubstantially similar footprint as the first inductor is provided. Thefirst inductor to provide an effective inductive load to circuitrycoupled to the first inductor. The second inductor comprises atransistor having a first terminal coupled to a first end of the secondinductor and a second terminal coupled to a second end of the secondinductor, the second inductor to affect the effective inductive loadprovided by the first inductor.

In accordance with another preferred embodiment of the presentinvention, an oscillator with extended frequency tuning range isprovided. The oscillator comprises a voltage controlled oscillator(VCO), the VCO is configured to generate an output signal at a frequencythat is dependent upon a magnitude of an input voltage signal and aneffective inductance of an inductive load, and a variable inductorcoupled to the VCO is provided. The variable inductor comprising aprimary inductor coupled to the VCO, the primary inductor to produce amagnetic field based upon a current flowing through the primaryinductor, resulting in an inductive load for the VCO, and a secondaryinductor magnetically coupled to the primary inductor, the secondaryinductor to affect the magnitude of the effective inductance of theprimary inductor.

In accordance with another preferred embodiment of the presentinvention, a method for designing an oscillator with extended frequencytuning range is provided. The method comprises determining a number ofinductance loops in the oscillator based upon the extended frequencytuning range, calculating a set of characteristics for each inductanceloop, simulating the performance of the oscillator, and fabricating theoscillator if the oscillator meets performance expectations.

An advantage of a preferred embodiment of the present invention is thatthe frequency tuning range of a RF/microwave LC oscillator is increasedwithout the use of multiple oscillator cores and LC tanks. Sincemultiple oscillator cores and LC tanks are not used, the LC oscillatorcan be kept to a minimum size, thereby decreasing the silicon footprintof the LC oscillator.

A further advantage of a preferred embodiment of the present inventionis that only an insignificant parasitic capacitance is added to the LCoscillator. Therefore, as the operating frequencies increase, theparasitic capacitance does not have an adverse affect on the LCoscillator.

Yet another advantage of a preferred embodiment of the present inventionis that multiple inductors can be added to the LC oscillator to providea wide frequency tuning range. For example, more than one inductor canbe added to the LC oscillator and these added inductors can be switchedin and out, either individually or in combination, to provide a widefrequency tuning range with potentially fine tuning adjustments.

The foregoing has outlined rather broadly the features and technicaladvantages of the present invention in order that the detaileddescription of the invention that follows may be better understood.Additional features and advantages of the invention will be describedhereinafter which form the subject of the claims of the invention. Itshould be appreciated by those skilled in the art that the conceptionand specific embodiments disclosed may be readily utilized as a basisfor modifying or designing other structures or processes for carryingout the same purposes of the present invention. It should also berealized by those skilled in the art that such equivalent constructionsdo not depart from the spirit and scope of the invention as set forth inthe appended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

For a more complete understanding of the present invention, and theadvantages thereof, reference is now made to the following descriptionstaken in conjunction with the accompanying drawings, in which:

FIG. 1 is a frequency diagram of an exemplary multi-band cellulartelephone;

FIGS. 2 a through 2 c are diagrams of a variable transformer along withseries and parallel equivalent circuits, according to a preferredembodiment of the present invention;

FIGS. 3 a through 3 c are diagram of several views of a variableinductor, wherein a single secondary inductor is used to change theinductance of the primary inductor, according to a preferred embodimentof the present invention;

FIGS. 4 a through 4 c are diagram of several views of a variableinductor, wherein a pair of secondary inductors are used to change theinductance of the primary inductor, according to a preferred embodimentof the present invention;

FIG. 5 is a diagram of a serial equivalent circuit of a variableinductor, according to a preferred embodiment of the present invention;

FIGS. 6 a and 6 b are diagrams of a circuit comprising a VCO and avariable inductor, wherein the variable inductor can help to increasethe frequency tuning range of the VCO, according to a preferredembodiment of the present invention;

FIGS. 7 a and 7 b are diagrams of a circuit comprising a VCO and avariable inductor, wherein the variable inductor and active componentscoupled to the variable inductor can help to further increase the tuningrange of the VCO, according to a preferred embodiment of the presentinvention; and

FIG. 8 is a flow diagram of a sequence of events for designing aRF/microwave oscillator with an extended frequency tuning range,according to a preferred embodiment of the present invention.

DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS

The making and using of the presently preferred embodiments arediscussed in detail below. It should be appreciated, however, that thepresent invention provides many applicable inventive concepts that canbe embodied in a wide variety of specific contexts. The specificembodiments discussed are merely illustrative of specific ways to makeand use the invention, and do not limit the scope of the invention.

The present invention will be described with respect to preferredembodiments in a specific context, namely a multi-standard cellulartelephone that needs to operate in multiple frequency ranges. Theinvention may also be applied, however, to other electronic devices,such as communications devices, that have a need to operate in multiplefrequency ranges.

With reference now to FIG. 1, there is shown a frequency diagram for anexemplary cellular telephone, wherein the cellular telephone is amulti-standard telephone. The frequency diagram shown in FIG. 1illustrates various frequency ranges wherein the cellular telephone isrequired to operate. As shown in FIG. 1, the frequency diagramillustrates the frequency ranges in use for a multi-standard cellulartelephone that is designed to be compatible with multiple GSM standardsbeing used throughout the world. Note that while the frequency diagramillustrates different GSM standards, the use of specific frequencyranges and communications standards, namely, GSM, should not beconstrued as being limiting to the spirit of the present invention. Forexample, the present invention can be equally applicable to the variousTDMA and CDMA communications standards in use throughout the world.Furthermore, the discussion of cellular telephones and cellulartelephone communications standards should also not be construed as beinglimiting to the spirit of the present invention to only cellulartelephones. The present invention can be applicable to other wired andwireless communications devices, such as tunable filters, wired localarea networks (LANs), wireless LANs such as Wi-Fi, Global PositioningSystem (GPS), walkie-talkies, two-way radios, satellite telephones,optical communications devices, clock recovery circuits, wireless inputdevices (mouse, keyboard, etc.), wireless entertainment systems, and soforth.

The frequency diagram shows two frequency bands, a low band 105 (rangingfrom 820 MHz to 960 MHz) and a high band 125 (ranging from 1.7 GHz to1.98 GHz). The low band 105 includes a GSM 850 band 110 and an EnhancedGlobal System for Mobile Communications (EGSM) 900 band 112 while thehigh band 125 includes a Digital Cellular System (DCS) 1800 band 130 anda Personal Communications Service (PCS) 1900 band 132. Within each band,there can be two frequency ranges that the cellular telephone should beable to tune to, a transmit range (TX) and a receive range (RX). Forexample, in the EGSM 900 band 112, a TX range 115 spans a frequencyrange of 880 MHz to 915 MHz and a RX range 117 spans a frequency rangeof 925 MHz to 960 MHz.

Due to the relatively wide frequency range of the low band 105 (136 MHz)and the high band 125 (280 MHz), it can be difficult for a single LCoscillator to span the entire frequency range. This may be due toconstraints such as a low-power voltage supply, device characteristicsof the varactor (limited C_(max)/C_(min) ratio), parasitic capacitanceof active and passive devices, additional parasitic capacitance due todummy metal patterns in deep-submicron CMOS process technology, largeoscillator signal swing decreasing an effective varactor C_(max)/C_(min)ratio, and so forth. Difficulties in creating a single LC oscillatorthat is capable of tuning the entire range of both the low band 105 andthe high band 125 are considered to be well understood by those ofordinary skill in the art of the present invention and will not bediscussed further.

While the design of a single LC oscillator that is capable of tuning theentire range of a wide frequency range can be difficult, if notimpossible, prior art techniques have proposed the creation of an LCoscillator that uses multiple oscillator cores and LC tanks. The use ofthe multiple oscillator cores and LC tanks can extend the tuning rangeof the LC oscillator so that the entire frequency range can be covered.The use of multiple oscillator cores and LC tanks have been proposed indocuments entitled “A 3.6 GHz Double Cross-Coupled Multivibrator VCOwith 1.6 GHz Tuning,” published August 2001 and “RF-CMOS Oscillatorswith Switched Tuning,” published 1998. Both documents are incorporatedherein by reference. However, the use of multiple oscillator cores andLC tanks can greatly increase the size of the LC oscillator. Anadditional prior art technique proposes the use of a CMOS switch toshort circuit a portion of an inductor to induce a shift in theoscillation frequency of the LC oscillator. The technique was describedin a document entitled “Demonstration of a Switched Resonator Concept ina Dual-Band Monolithic CMOS LC-Tuned VCO,” published 2001, which isherein incorporated by reference. Unfortunately, the CMOS switch can bea source of significant parasitic capacitance, which can present aproblem as the operating frequencies increase due to the relativelysmall capacitance of the LC oscillator.

With reference now to FIGS. 2 a through 2 c, there are shown diagramsillustrating a variable transformer along with series and parallelequivalent circuits, according to a preferred embodiment of the presentinvention. The diagram shown in FIG. 2 a illustrates a view of avariable transformer 200 wherein the variable transformer 200 comprisesa primary inductor 205 with an inductance of L_(PRIM) and a secondaryinductor 210 with an inductance L_(SEC). The primary inductor 205 havinga pair of terminals, a primary_plus terminal (labeled “PRIM_P”) and aprimary_minus terminal (labeled “PRIM_M”), and the secondary inductor210 having a pair of terminals, a secondary_plus terminal (labeled“SEC_P”) and a secondary_minus terminal (labeled “SEC_M”). The secondaryinductor 210 can be effectively switched on and off via a switch 215.According to a preferred embodiment of the present invention, thesecondary inductor 210 is switched on when the switch 215 is in a closedstate and the secondary inductor 210 is switched off when the switch 215is in an open state. When the secondary inductor 210 is switched on, theinductance of the secondary inductor 210 can have an effect upon thefirst inductor 205, changing the effective inductance of the primaryinductor 205, and thereby changing the tuning frequency of the LCoscillator. When the secondary inductor 210 is switched off, thesecondary inductor 210 is effectively removed from the LC oscillator andthe LC oscillator behaves as if it had only the primary inductor 205.

The effect of the secondary inductor 210 on the primary inductor 205 canbe dependent upon factors that include the inductance of the secondaryinductor 210, the coupling factor between the primary inductor 205 andthe secondary inductor 210 (which can include the separation distancebetween the inductors, the material between the inductors, and soforth), and so on. Mutual inductance is considered to be well understoodby those of ordinary skill in the art of the present invention and willnot be discussed in further detail herein.

The diagram shown in FIG. 2 b illustrates a serial equivalent circuit220 of the variable inductor 200. In the serial equivalent circuit 220,the primary inductor 205 can be replaced by an equivalent first inductor225 and a first serial resistor 227 while the secondary inductor 210 canbe replaced by an equivalent second inductor 235 and a second serialresistor 237. The first serial resistor 227 and the second serialresistor 237 can be representative of parasitic resistances seen in theprimary inductor 205 and the secondary inductor 210. Similarly, thediagram shown in FIG. 2 c illustrates a parallel equivalent circuit 240of the variable inductor 200, wherein the primary inductor 205 can bereplaced by an equivalent first inductor 245 and a first parallelresistor 247 while the secondary inductor 210 can be replaced by anequivalent second inductor 255 and a second parallel resistor 257. Notethat if the secondary inductor 210 has a high quality factor, then theresistance of the second parallel resistor 257 would be high. Thetransformed equivalent parallel resistance (the second parallel resistor257, as seen at the primary inductor 205) would therefore also be high,resulting in a minimum overall impact on the primary inductor 205.

When active components are not attached to the terminals of thesecondary inductor 210, SEC_P and SEC_M, to actively control a currentflowing in the secondary inductor 210, then due to the fact that thesecondary inductor 210 operates with current that is magneticallyinduced by the primary inductor 205, the overall effective magnetic fluxseen by the primary inductor 205 is reduced when compared to themagnetic flux seen by the primary inductor 205 if it were operatingalone (or if the secondary inductor 210 is off). Therefore, theeffective inductance between the two terminals of the primary inductor205, PRIM_P and PRIM_M, is decreased. Additionally, the effectivequality factor of the primary inductor 205 can be reduced by the samepercentage as that of the inductance of the primary inductor 205 if thesecondary inductor 210 has a high quality factor. Therefore, when thevariable inductor 200 is used in conjunction with an LC oscillator, theoscillation frequency of the LC oscillator would be higher when theswitch 215 is closed and the oscillation frequency of the LC oscillatorwould be substantially unaffected when the switch 215 is open. Note thatthe variable transformer 200 can be optimized for the most sensitivefrequency spot (where there is the highest quality (Q) requirement)corresponding to a frequency specified in the communications standardwith the highest Q requirement. Other desired parameters can also beused to optimize the design of the variable transformer 200.

With reference now to FIGS. 3 a through 3 c, there are shown diagrams ofdiffering views of a variable inductor 300, wherein a single secondaryinductor is used to change the inductance of a primary inductor,according to a preferred embodiment of the present invention. Note thatfor clarity, the conductors forming the inductors are illustrated asthin lines connecting adjacent vertices of the inductors. The diagramshown in FIG. 3 a illustrates an isometric view of the variable inductor300 that may be formed on a semiconductor substrate. Again, for clarity,the semiconductor substrate is not shown, nor are any other componentsof an integrated circuit. A primary inductor 305 is shown formed above asecondary inductor 310. Note that since the primary inductor 305 isformed above the secondary inductor 310, there is little surface areapenalty from the use of the secondary inductor 310. Additionally, inorder to increase Q of the primary inductor 305, multiple layers abovesemiconductor substrate are electrically connected in parallel to reducethe parasitic serial resistance except at the cross-overs that some ofthe layers are on top of the other (shown in FIG. 3 a as portionsextending below the surface of the primary inductor 305). Furthermore,although the secondary inductor 310 is shown as being formed below theprimary inductor 305, the secondary inductor 310 may also be formedabove the primary inductor 305, partially overlapping the primaryinductor 305, outside (or inside) the primary inductor 305, or alongside the primary inductor 305. In the last two cases (outside/inside andalong side), the secondary inductor 310 may be formed on the same layeras the primary inductor 305.

As discussed previously, the effect of the secondary inductor 310 uponthe primary inductor 305 can be a function of a separation between theprimary inductor 305 and the secondary inductor 310 as well as the sizeand shape of the secondary inductor 310. The secondary inductor 310 isshown in FIG. 3 a as being a closed loop. Not shown is a switch that canbe used to break the loop in the secondary inductor 310. The switch canbe located anywhere in the secondary inductor 310, such as along aportion highlighted in region 315. According to a preferred embodimentof the present invention, the switch can be opened or closed to affectthe effective inductance of the primary inductor 305. For example, ifthe switch is closed, then the effective inductance of the primaryinductor 305 can be reduced and if the switch is opened, then theeffective inductance of the primary inductor 300 remains unaffected.Furthermore, the resistance of the switch itself can be controlled tolimit the magnetically induced current flowing in the secondary inductor310 to provide another degree of freedom that can be used for varyingthe effective inductance of the primary inductor 305.

The figure shown in FIG. 3 b illustrates a top view of the variableinductor 300. The top view shows that the primary inductor 305 issmaller in area than the secondary inductor 310. Note however that dueto the winding of the conductor in the primary inductor, the inductanceof the primary inductor 305 may be larger than the inductance of thesecondary inductor 305. The figure shown in FIG. 3 c illustrates a sideview of the variable inductor 300. The side view shows that the primaryinductor 305 is clearly separated from the secondary inductor 310 by athickness of an interlayer dielectric between the layers forming the twoinductors. Note that the portion of the variable inductor 300 shown inFIG. 3 c does not illustrate the portions of the electrical conductorsmaking up the inductors that lay in layers other than the primarylayers.

With reference now to FIGS. 4 a through 4 c, there are shown diagrams ofdiffering views of a variable inductor 400, wherein a pair of secondaryinductors are used to change the inductance of a primary inductor,according to a preferred embodiment of the present invention. Thediagram shown in FIG. 4 a illustrates an isometric view of the variableinductor 400 that may be formed on a semiconductor substrate. Thevariable inductor 400 comprises a primary inductor 405 and a pair ofsecondary inductors, a first secondary inductor 410 and a secondsecondary inductor 415. Once again, since the pair of secondaryinductors are formed beneath the primary inductor 405, there is littlesurface area penalty incurred from the use of the secondary inductors.Note that the first secondary inductor 410 and the second secondaryinductor 415 can be formed on different layers on the semiconductorsubstrate. Alternatively, with sizing restrictions in place to ensurethat electrical contact is not made, the first secondary inductor 410and the second secondary inductor 415 can be formed on a single layer.The present invention is not limited to the use of one secondaryinductor or a pair of secondary inductors, therefore, the discussion ofa single secondary inductor (as shown in FIGS. 3 a through 3 c) and of apair of secondary inductors should not be construed as limiting thespirit of the present invention to the use of one or two secondaryinductors. As stated earlier, the layers of secondary inductors 410 and415 are not limited to be beneath the primary inductor layer.

Switches that can be used to control the state of the first secondaryinductor 410 and the second secondary inductor 415 are not shown in FIG.4 a. As discussed above, the state of the switches can be used to affectthe effective inductance of the primary inductor 405 and can be usedindividually or in conjunction with one another. For discussionpurposes, let a switch controlling the state of the first secondaryinductor 410 be referred to as ‘switch one’ while ‘switch two’ be areference for a switch controlling the state of the second secondaryinductor 415, then at a given instance of time, the state of switch oneand switch two can be as follows: switch one OPEN and switch twoOPEN—yielding an effective inductance at the primary inductor 405substantially equal to the inductance of the primary inductor 405;switch one CLOSED and switch two OPEN—yielding an effective inductanceat the primary inductor 405 that is a function of only the inductancesof the first secondary inductor 410 and the primary inductor 405 as wellas the coupling coefficient between the first secondary inductor 410 andthe primary inductor 405; switch one OPEN and switch two CLOSED—yieldingan effective inductance at the primary inductor 405 that is a functionof only the inductances of the second secondary inductor 415 and theprimary inductor 405 as well as the coupling coefficient between thesecond secondary inductor 415 and the primary inductor 405; switch oneCLOSED and switch two CLOSED—yielding an effective inductance at theprimary inductor 405 that is a function of the inductances of both thefirst secondary inductor 410 and the second secondary inductor 415 andthe primary inductor 405 as well as the coupling coefficients among thefirst secondary inductor 410 and the second secondary inductor 415 andthe primary inductor 405.

The figure shown in FIG. 4 b illustrates a top view of the variableinductor 400. The top view shows that the first secondary inductor 410is smaller physically than the second secondary inductor 415. Notehowever, it is possible to have a variable inductor with a secondsecondary inductor that is smaller physically than the first secondaryinductor. Although shown in FIG. 4 b to have similar shape, the shape ofthe first secondary inductor 410 can be different from the shape of thesecond secondary inductor 415. This is especially true when thesecondary inductors are formed on different layers on the semiconductorsubstrate. However, when the secondary inductors are formed on a singlelayer of the semiconductor substrate, the shapes and sizes of thesecondary inductors may be restricted due to constraints such as thesecondary inductors must be electrically disjoint and that a majority ofthe secondary inductors must lie beneath the primary inductor 405. Thefigure shown in FIG. 4 c illustrates a side view of the variableinductor 400. The side view shows that the primary inductor 405 isclearly separated from the first secondary inductor 410 and the secondsecondary inductor 415 and that the first secondary inductor 410 isclosest to the primary inductor 405. Once again, note that the portionof the variable inductor 400 shown in FIG. 4 c does not illustrateportions of the electrical conductors making up the inductors that layin layers other than the primary layers.

With reference now to FIG. 5, there is shown a diagram illustrating aserial equivalent circuit 500 of a variable inductor, according to apreferred embodiment of the present invention. Since the variableinductor can be implemented as a pair of transformers (primary inductor205 and secondary inductor 210), the variable inductor can be shown ashaving four inductors (shown in FIG. 5 in their serial equivalentcircuit form) to help understand the concept, especially when it isimplemented with a differential circuit: a first primary inductor 505and a second primary inductor 510 making up the primary inductor 205 anda first secondary inductor 515 and a second secondary inductor 520making up the secondary inductor 210. As discussed previously, in theserial equivalent form, an inductor can be represented as an inductorwith a parasitic resistance. The parasitic resistances for the fourinductors are shown in FIG. 5 as resistors 507, 512, 517, and 522respectively.

Parasitic capacitances can distributively exist between the conductorsand are usually represented as lumped capacitors between terminals (theplus, center, and minus terminals of each inductor) of the primaryinductor 205 and the secondary inductor 210 in the variable inductorsuch as capacitors 525 and 527. In addition to the parasitic resistancesin the four inductors (resistors 507, 512, 517, and 522) and theparasitic capacitances between conductors (capacitors 525 and 527),parasitic capacitances and resistances also distributively exist betweenthe conductors and an electrical AC ground, which are usuallyrepresented as lumped capacitors and resistors between each terminal ofthe primary inductor 205 and electrical AC ground as well as betweeneach terminal of the secondary inductor 210 and electrical AC groundsuch as capacitors 530 and 531 and resistors 532 and 533 (note that inFIG. 5, capacitors and resistors representing parasitic capacitances andresistances are shown for each terminal of the primary inductor 205 andthe secondary inductor 210 but are not referenced).

Coupling between the inductors (inductors 505, 510, 515, and 520) can bemodeled as six (6) individual coupling factors. A first coupling factor535 models a coupling between the inductor 505 and the inductor 510(both of the primary inductor 205), while a second coupling factor 540models a coupling between the inductor 515 and the inductor 520 (both ofthe secondary inductor 210). The remaining four coupling factors modelcoupling between the primary inductor 205 and the secondary inductor210. A third coupling factor 545 models a coupling between the inductor505 and the inductor 515, a fourth coupling factor 550 models a couplingbetween the inductor 505 and the inductor 520, a fifth coupling factor555 models a coupling between the inductor 510 and the inductor 520, anda sixth coupling factor 560 models a coupling between the inductor 510and the inductor 515.

With reference now to FIGS. 6 a and 6 b, there are shown diagramsillustrating views of a circuit 600 comprising a voltage-controlledoscillator (VCO) 605 and a variable inductor 610, wherein the variableinductor 610 can help to increase the tuning range of the VCO 605,according to a preferred embodiment of the present invention. Thediagram shown in FIG. 6 a illustrates a high-level functional view ofthe circuit 600. The circuit 600, shown in FIGS. 6 a and 6 b, is a LCoscillator and makes use of the variable inductor 610 to increase itsfrequency tuning range without having to use multiple oscillator coresand LC tanks. The VCO 605 can be coupled to the variable inductor 610 bya pair of signals, oscillator plus “OSCP” and oscillator minus “OSCM.”The inductance seen across the OSCP and OSCM signals can have an effectupon the frequency of the output of the VCO 605.

The diagram shown in FIG. 6 b illustrates a detailed schematic of aparticular implementation of the circuit 600. The detailed schematic ofthe VCO 605 shows a typical VCO and will not be described herein. Thedetailed schematic of the variable inductor 620 illustrates the primaryinductor 205 and the secondary inductor 210 shown as a four inductormodel, similar to the serial model shown in FIG. 5 without parasiticresistances, parasitic capacitances, and coupling factors to helpmaintain simplicity. A transistor 612, preferably a N-type metal oxidesemiconductor (NMOS) transistor although a P-type metal oxidesemiconductor (PMOS) transistor can also be used, can be used to controlthe state of the secondary inductor 210. Additionally, a bi-polarjunction transistor (BJT) can also be used to control the state of thesecondary inductor 210 by functioning as a switch. The transistor 612,operating as a switch, can be controlled by a signal at its gateterminal, displayed in FIG. 6 b as signal “VCTL.”

The oscillator plus “OSCP” and oscillator minus “OSCM” signals from theVCO 605 can be coupled to the primary plus “PRIM_P” and the primaryminus “PRIM_M” terminals of the primary inductor 205. Additionally, thecenter terminal “PRIM_C” of the primary inductor 205 can be coupled to avoltage supply providing the current to operate the VCO 605.

Using engineering design tools, the circuit 600 was simulated using themodel of the variable inductor 610 shown in FIG. 5 with the followingparameters: the first primary inductor 505 was 0.5988 nH, the secondprimary inductor 510 was 0.5988 nH, the first secondary inductor 515 was0.2389 nH, the second secondary inductor 520 was 0.2389 nH, the firstthrough sixth coupling factors (coupling factors 535, 540, 545, 550,555, and 560) were 0.618371, −0.320204, −0.291226, −0.304592, −0.274701,and −0.122267 respectively, the parasitic series resistances are 1.17125ohms for resistors 507 and 512 and 2 ohms for resistors 517 and 522, theparasitic capacitance (such as capacitor 525) between the primaryinductor 205 and the secondary inductor 210 was 56.892 pf each.Simulations show that with the transistor 612 open, the circuit 600produced an oscillating frequency of 3.371 GHz and an oscillatingfrequency of 3.787 GHz when the transistor 612 is closed. The VCO 605,alone, provides a frequency tuning range of approximately 800 MHz.However, when used in conjunction with the variable inductor 610, thefrequency tuning range increases to approximately 1200 MHz, or a 50percent increase.

According to a preferred embodiment of the present invention, thefrequency tuning range can be further increased by connecting the centerterminal “SEC_C” (not labeled in FIG. 6 b) of the secondary inductor 210to electrical power or ground (i.e. an AC ground) with a large resistorand through redesigning the second inductor 210. Simulation results showthat the phase noise of the circuit 600 is approximately 9 dB below thatof the VCO 605 due to the lack of optimization of the second inductor210.

Although the diagram in FIG. 6 b illustrates a single secondary inductor210, multiple inductors can be used in place of the single inductor. Themultiple inductors can be used separately from each other to alter theeffective inductance of the primary inductor 205 or they can be used incombination with each other to provide a greater change on the effectiveinductance of the primary inductor 205.

With reference now to FIGS. 7 a and 7 b, there are shown diagramsillustrating views of a circuit 700 comprising a voltage-controlledoscillator (VCO) 605 and a variable inductor 610, wherein the variableinductor 610 and active components coupled to the variable inductor 610can help to further increase the tuning range of the VCO 605, accordingto a preferred embodiment of the present invention. When active devicesare not coupled to the secondary inductor 210, the variable inductor 610can change the oscillating frequency of the VCO 605 by decreasing theeffective inductance of the primary inductor 205, this results in anincrease in the oscillating frequency. However, when active devices arecoupled to the secondary inductor 210, it can be possible to cause anincrease in the effective inductance of the primary inductor 205,resulting in a decrease in the oscillating frequency.

The diagram shown in FIG. 7 a illustrates a high-level functional viewof the circuit 700. The circuit 700 includes the VCO 605 and thevariable inductor 610 coupled together via a pair of signals, “OSCP” and“OSCM,” similar to the circuit 600 (FIG. 6 a). However, two transistors705, preferably NMOS transistors although PMOS transistors can also beused, also couple the VCO 605 to the variable inductor 610. Again, BJTtransistors can be used in place of the field-effect transistors (NMOSor PMOS transistors). The two transistors 705 are controlled by the“OSCP” and “OSCM” signals produced by the VCO 605. The transistor thatis controlled by the “OSCP” signal can have a source/drain terminalcoupled to the plus terminal “SEC_P” of the secondary inductor 210 whilethe transistor that is controlled by the “OSCM” signal can have asource/drain terminal coupled to the minus terminal “SEC_M” of thesecondary inductor 210.

The presence of the active components (the pair of transistors 705) canfunction as positive feedback switches to force current in the secondaryinductor 210 to flow in an opposite direction to the current flowing inthe primary inductor 205. This effect can result in a decrease in theeffective inductance of the primary inductor 205 and result in anincrease in the oscillating frequency of the circuit 700. Alternatively,the current in the secondary inductor 210 can be forced to flow in thesame direction as the current flowing in the primary inductor 205 tofurther enhance the effective inductance of the primary inductor 205 andpossibly result in an increase in the quality (Q) factor and a decreasein the oscillating frequency. Note that different arrangements of activecomponents can be used in place of the pair of transistors 705 to yieldsimilar (or enhanced) effects. Note that the inclusion of the pair oftransistors 705 will incur a very small area penalty due to the smallsize requirements of the pair of transistors 705.

The diagram shown in FIG. 7 b illustrates a detailed schematic of aparticular implementation of the circuit 700. The schematic of the VCO605 is similar to the schematic of the VCO 605 shown in FIG. 6 b. Theschematic of the variable inductor 610 shows that the primary inductor205 is also similar to the configuration of the primary inductor 205shown in FIG. 6 b. However, with the secondary inductor 210, rather thanhaving a transistor (transistor 612 (FIG. 6 b)) controlling the state ofthe secondary inductor 210, the plus terminal and the minus terminal ofthe secondary inductor 210 are coupled to the pair of transistors 705while the center terminal can be coupled to electrical ground, a voltagesupply or to the center terminal of the primary inductor 205. Asdiscussed above, the single secondary inductor 205 can be replaced withmultiple inductors, with each inductor being coupled to the VCO 605 by apair of transistors.

According to yet another preferred embodiment of the present invention,it can be possible to combine the design of the secondary inductor 205illustrated in FIG. 6 b with that shown in FIG. 7 b. For example, in asecondary inductor with two inductors, a first inductor can beconfigured as shown in FIG. 6 b while a second inductor can beconfigured as shown in FIG. 7 b.

According to yet another preferred embodiment of the present invention,configurations in FIG. 6 b and FIG. 7 b can be further combined byadding a switch 612 between SEC_P and SEC_M of the secondary inductor inFIG. 7 b.

With reference now to FIG. 8, there is shown a flow diagram illustratinga sequence of events 800 for designing a RF/microwave oscillator with anextended frequency tuning range, according to a preferred embodiment ofthe present invention. The sequence of events 800 illustrates anexemplary design for a RF/microwave oscillator with an extendedfrequency tuning range that makes use of a variable inductor. Thesequence of events 800 can begin with a specification of a number offrequency ranges needed (block 805). The number of frequency rangesneeded can be dependent upon factors such as the width of the overallfrequency range that needs to be spanned, the frequency tuning range ofthe oscillator that will be used in the design, the desired quality (Q)of the inductor and LC tank, and so forth. After the number of frequencyranges have been determined, then a number of inductance loops needed iscalculated (block 810). The number of inductance loops needed can bedependent upon a design selected for the variable inductor. For example,if simple inductance loops using switches to regulate the state of theloops is used, then the number of inductance loops may be equal to thenumber of frequency ranges. A more sophisticated inductance loop design,such as one using active components to change the effective inductancein several ways or if inductance loops can be used in conjunction withone another, then the number of inductance loops can be less than thenumber of frequency ranges.

Then, based upon the frequency ranges, the number of inductance loops,the design of the oscillator, and so on, the characteristics of theinductance loops can be calculated (block 815). The characteristics ofthe inductance loops can include but are not limited to the size of theinductance loops, the shape of the inductance loops, the location of theinductance loops, the separation of the inductance loops in relation toa primary inductor, and so forth. Additionally, the ON resistance of theswitches used to regulate the state of the loops, wherein if a FET isused as a switch, the ON resistance can be calculated based on gatewidth and gate length, while if a BJT is used as a switch, the ONresistance can be calculated based on silicon area of the collector andthe emitter of the BJT. With the characteristics of the inductance loopsdetermined, the variable inductor can be laid out along with theremainder of the oscillator (block 820). Then, using engineering designtools, it can be possible to simulate the performance of the oscillator(block 825). The simulation of the oscillator can be used to verifyperformance before the expensive fabrication of the oscillator takesplace. If the simulation shows that the oscillator performs as desired(block 830), then the oscillator is ready for fabrication (block 835).If the simulation shows that the oscillator does not perform as desired,then the design of the variable inductor may need to be revised.

Although the present invention and its advantages have been described indetail, it should be understood that various changes, substitutions andalterations can be made herein without departing from the spirit andscope of the invention as defined by the appended claims.

Moreover, the scope of the present application is not intended to belimited to the particular embodiments of the process, machine,manufacture, composition of matter, means, methods and steps describedin the specification. As one of ordinary skill in the art will readilyappreciate from the disclosure of the present invention, processes,machines, manufacture, compositions of matter, means, methods, or steps,presently existing or later to be developed, that perform substantiallythe same function or achieve substantially the same result as thecorresponding embodiments described herein may be utilized according tothe present invention. Accordingly, the appended claims are intended toinclude within their scope such processes, machines, manufacture,compositions of matter, means, methods, or steps.

1. An integrated circuit comprising: a semiconductor substrate; a firstinductor disposed over the semiconductor substrate and comprising afirst terminal and a second terminal, wherein the first inductorprovides an effective inductive load to circuitry coupled to the firstand second terminals of the first inductor; and a plurality of secondinductors disposed on the semiconductor substrate and comprising a firstterminal and a second terminal, the plurality of second inductorsadjacent to the first inductor so that the plurality of second inductorsare magnetically coupled to the first inductor, wherein each of saidplurality of second inductors has a corresponding variable impedanceelement coupled to the first and second terminal of each said secondinductor, and wherein each corresponding variable impedance elementcomprises a transistor.
 2. The integrated circuit of claim 1, whereinthe transistor of each variable impedance element comprising a firstterminal coupled to the first terminal of the corresponding secondinductor, a second terminal coupled to the second terminal of thecorresponding second inductor, and a control terminal, and wherein acontrol signal line is coupled to the control terminal of thetransistor, wherein the control signal line affects the effectiveinductive load provided to the circuitry coupled to the first and secondterminals of the first inductor.
 3. The integrated circuit of claim 1,wherein the corresponding second inductor is formed above, below,inside, outside or along side the first inductor.
 4. An oscillatorcomprising: a voltage controlled oscillator (VCO), the VCO is configuredto generate an output signal at a frequency that is dependent upon amagnitude of an input voltage signal and an effective inductance of a,inductive load; a variable inductor coupled to the VCO, the variableinductor comprising a primary inductor coupled to the VCO, the primaryinductor to produce a magnetic field based upon a current flowingthrough the primary inductor, resulting in an inductive load for theVCO; and a secondary inductor magnetically coupled to the primaryinductor, the secondary inductor to affect the magnitude of theeffective inductance of the primary inductor, wherein the secondaryinductor comprises: a conductive element, wherein the conductive elementhaving a first end and a second end; and a switch having a firstterminal coupled to the first end of the conductive element and a secondterminal coupled to the second end of the conductive element, the switchcontrol the state of the secondary inductor.
 5. The oscillator of claim4, wherein the inductive load is the primary inductor.
 6. The oscillatorof claim 4, wherein the switch is a transistor, and wherein thetransistor is a field-effect transistor (FET), wherein the firstterminal is a first drain/source terminal of the FET and the secondterminal is a second drain/source terminal of the FET.
 7. The oscillatorof claim 4, wherein the switch is a transistor, and wherein thetransistor is a bi-polar junction transistor (BJT), wherein the firstterminal is a first collector/emitter terminal of the BJT and the secondterminal is a second collector/emitter terminal of the BJT.
 8. Theoscillator of claim 4, wherein the secondary inductor is either openedor closed depending upon a state of the switch.
 9. The oscillator ofclaim 4, wherein the secondary inductor comprises a plurality ofconductive elements, wherein each conductive element has a first end anda second end, and wherein each conductive element is coupled to aswitch.
 10. The oscillator of claim 4, wherein the secondary inductorcomprises two terminals, the oscillator further comprising a pair ofactive components, the first active component coupled in between the VCOand a first terminal of the secondary inductor and the second activecomponent coupled in between the VCO and a second terminal of thesecondary inductor.
 11. The oscillator of claim 10, wherein the pair ofactive components are transistors, and wherein the transistors arefield-effect transistors (FET), wherein each FET has a gate terminal,and wherein a gate terminal of the first FET is coupled to a firstcontrol signal provided by the VCO and a gate terminal of the second FETis coupled to a second control signal provided by the VCO.
 12. Theoscillator of claim 10, wherein the pair of active components aretransistors, and wherein the transistors are bi-polar junctiontransistors (BJT), wherein each BJT having a base terminal, and whereina base terminal of the first BJT is coupled to a first control signalprovided by the VCO and a base terminal of the second BJT is coupled toa second control signal provided by the VCO.